As an example of such tool holding structure described above, e.g. Patent Document 1 discloses a structure wherein a pin insertion hole is provided as an inner fitting portion in an end face of a chuck portion (disclosed as a chuck cylinder in the above document) to extend through this end face. With this tool holding structure, in the chuck portion, a fitting member (disclosed as a lock pin in the document) is fitted from the outside of the chuck portion to the inner fitting portion, such that the fitting member is exposed in an inner space of the chuck portion. On the other hand, the shank portion includes, in its outer circumference, a cylindrical outer fitting portion (disclosed as a pin engaging groove in the document) to which the fitting member is to be fitted.
According to the above-described arrangement of Patent Document 1, after the fitting member is inserted to the inner fitting portion of the chuck portion, the chuck portion is heated to be expanded, into which the shank portion is inserted and then cooled together. Thus, with fitting between the chuck portion and the shank portion as well as fitting between the outer fitting portion and the fitting member through shrink-fits, the cutting tool is mounted. With this arrangement, when the cutting tool is exposed to application thereto of a force which tends to slip off this tool, the fitting between the outer fitting portion (pin engaging groove) of the shank portion and the fitting member prevents slip-out. Further, when the cutting tool tends to be rotated relative to the chuck portion, the fitting member prevents such relative rotation of the cutting tool relative to the chuck portion.
As another tool holding structure, Patent Document 2 discloses a structure wherein a rotatably driven base body (corresponding to a chuck portion) defines an inner space to which a shank portion of a drill bit can be inserted. In this inner space, there is formed an engaging protrusion (corresponding to a fitting member) which protrudes to the inner side of the space; whereas, in an outer face of the shank portion of the drill bit, there are formed a guide groove extending along a longitudinal direction of the drill bit, and an engaging groove disposed parallel with the guide groove and having a predetermined length whose intermediate portion is continuous with the guide groove. Further, in the inner space, there is provided a spring configured to urge the drill bit in a push-out direction.
With the above-described arrangement disclosed in Patent Document 2, for attaching the drill bit to the base body, the shank portion of this drill bit will be inserted with keeping the guide groove of the drill bit in positional alignment with the base body. Thereafter, the drill bit will be rotated to bring the engaging protrusion into the engaging groove, upon which the drill bit will be displaced in the protruding direction by the urging force of the spring, and the engaging protrusion will come into engagement with an end portion of the engaging groove. With this, slip-out of the drill bit from the base body is restrained and also transmission of a rotational torque from the body portion to the drill bit becomes possible.